Dust Evolution in Protoplanetary Discs and the Formation of Planetesimals

ABSTRACT The time-scales on which astronomical dust grows remain poorly understood, with important consequences for our understanding of processes like circumstellar disc evolution and planet formation. A number of post-asymptotic giant branch (AGB) stars are found to host optically thick, dust- and gas-rich circumstellar discs in Keplerian orbits. These discs exhibit evidence of dust evolution, similar to protoplanetary discs; however, since post-AGB discs have substantially shorter lifetimes than protoplanetary discs, they may provide new insights on the grain-growth process. We examine a sample of post-AGB stars with discs to determine the far-infrared and sub-millimetre spectral index by homogeneously fitting a sample of data from Herschel, the Submillimeter Array (SMA), and the literature. We find that grain growth to at least hundreds of micrometres is ubiquitous in these systems, and that the distribution of spectral indices is more similar to that of protoplanetary discs than debris discs. No correlation is found with the mid-infrared colours of the discs, implying that grain growth occurs independently of the disc structure in post-AGB discs. We infer that grain growth to ∼millimetre sizes must occur on time-scales <<105 yr, perhaps by orders of magnitude, as the lifetimes of these discs are expected to be ≲105 yr and all objects have converged to the same state. This growth time-scale is short compared to the results of models for protoplanetary discs including fragmentation and may provide new constraints on the physics of grain growth.

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Gas versus dust sizes of protoplanetary discs: effects of dust evolution

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834723 ◽

2019 ◽

Vol 629 ◽

pp. A79 ◽

Cited By ~ 16

Author(s):

L. Trapman ◽

S. Facchini ◽

M. R. Hogerheijde ◽

E. F. van Dishoeck ◽

S. Bruderer

Keyword(s):

Grain Growth ◽

Optical Depth ◽

Outer Radius ◽

Size Difference ◽

Clear Sign ◽

Thermochemical Model ◽

High Fraction ◽

Space Dust ◽

Dust Evolution ◽

Protoplanetary Discs

Context. The extent of the gas in protoplanetary discs is observed to be universally larger than the extent of the dust. This is often attributed to radial drift and grain growth of the millimetre grains, but line optical depth produces a similar observational signature. Aims. We investigate in which parts of the disc structure parameter space dust evolution and line optical depth are the dominant drivers of the observed gas and dust size difference. Methods. Using the thermochemical model DALI with dust evolution included we ran a grid of models aimed at reproducing the observed gas and dust size dichotomy. Results. The relation between Rdust and dust evolution is non-monotonic and depends on the disc structure. The quantity Rgas is directly related to the radius where the CO column density drops below 1015 cm−2 and CO becomes photodissociated; Rgas is not affected by dust evolution but scales with the total CO content of the disc. While these cases are rare in current observations, Rgas/Rdust > 4 is a clear sign of dust evolution and radial drift in discs. For discs with a smaller Rgas/Rdust, identifying dust evolution from Rgas/Rdust requires modelling the disc structure including the total CO content. To minimize the uncertainties due to observational factors requires FWHMbeam < 1× the characteristic radius and a peak S/N > 10 on the 12CO emission moment zero map. For the dust outer radius to enclose most of the disc mass, it should be defined using a high fraction (90–95%) of the total flux. For the gas, any radius enclosing >60% of the 12CO flux contains most of the disc mass. Conclusions. To distinguish radial drift and grain growth from line optical depth effects based on size ratios requires discs to be observed at high enough angular resolution and the disc structure should to be modelled to account for the total CO content of the disc.

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Self-induced dust traps around snow lines in protoplanetary discs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3444 ◽

2019 ◽

Vol 492 (1) ◽

pp. 210-222 ◽

Cited By ~ 3

Author(s):

Arnaud Vericel ◽

Jean-François Gonzalez

Keyword(s):

Carbon Monoxide ◽

The Self ◽

Dust Particles ◽

Parameter Study ◽

Volatile Species ◽

Dust Trap ◽

Multiple Observations ◽

Snow Line ◽

Dust Evolution ◽

Protoplanetary Discs

ABSTRACT Dust particles need to grow efficiently from micrometre sizes to thousands of kilometres to form planets. With the growth of millimetre to metre sizes being hindered by a number of barriers, the recent discovery that dust evolution is able to create ‘self-induced’ dust traps shows promises. The condensation and sublimation of volatile species at certain locations, called snow lines, are also thought to be important parts of planet formation scenarios. Given that dust sticking properties change across a snow line, this raises the question: how do snow lines affect the self-induced dust trap formation mechanism? The question is particularly relevant with the multiple observations of the carbon monoxide (CO) snow line in protoplanetary discs, since its effect on dust growth and dynamics is yet to be understood. In this paper, we present the effects of snow lines in general on the formation of self-induced dust traps in a parameter study, and then focus on the CO snow line. We find that for a range of parameters, a dust trap forms at the snow line where the dust accumulates and slowly grows, as found for the water snow line in a previous work. We also find that, depending on the grains’ sticking properties on either side of the CO snow line, it could be either a starting or braking point for dust growth and drift. This could provide clues to understand the link between dust distributions and snow lines in protoplanetary disc observations.

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Imaging [CI] around HD 131835: reinterpreting young debris discs with protoplanetary disc levels of CO gas as shielded secondary discs

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/sty2923 ◽

2018 ◽

Cited By ~ 11

Author(s):

Quentin Kral ◽

Sebastian Marino ◽

Mark C Wyatt ◽

Mihkel Kama ◽

Luca Matrá

Keyword(s):

Steady State ◽

Production Rate ◽

Analytic Model ◽

Dust Evolution ◽

Co Gas ◽

Protoplanetary Discs

Abstract Despite being >10Myr, there are ∼10 debris discs with as much CO gas as in protoplanetary discs. Such discs have been assumed to be “hybrid”, i.e., with secondary dust but primordial gas. Here we show that both the dust and gas in such systems could instead be secondary, with the high CO content caused by accumulation of neutral carbon (C0) that shields CO from photodissociating; i.e., these could be “shielded secondary discs”. New ALMA observations are presented of HD131835 that detect ∼3 × 10−3 M⊕ of C0, the majority 40-200au from the star, in sufficient quantity to shield the previously detected CO. A simple semi-analytic model for the evolution of CO, C and O originating in a volatile-rich planetesimal belt shows how CO shielding becomes important when the viscous evolution is slow (low α parameter) and/or the CO production rate is high. Shielding by C0 may also cause the CO content to reach levels at which CO self-shields, and the gas disc may become massive enough to affect the dust evolution. Application to the HD 131835 observations shows these can be explained if α ∼ 10−3; an inner cavity in C0 and CO may also mean the system has yet to reach steady state. Application to other debris discs with high CO content finds general agreement for α = 10−3 to 0.1. The shielded secondary nature of these gas discs can be tested by searching for C0, as well as CN, N2 and CH+, which are also expected to be shielded by C0.

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Pebble drift and planetesimal formation in protoplanetary discs with embedded planets

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937037 ◽

2020 ◽

Vol 635 ◽

pp. A110 ◽

Cited By ~ 4

Author(s):

Linn E. J. Eriksson ◽

Anders Johansen ◽

Beibei Liu

Keyword(s):

Spatial Distribution ◽

Time Scale ◽

Particle Sizes ◽

Dust Trapping ◽

Nominal Model ◽

Streaming Instability ◽

Dust Evolution ◽

Strong Depletion ◽

Protoplanetary Discs

Nearly axisymmetric gaps and rings are commonly observed in protoplanetary discs. The leading theory regarding the origin of these patterns is that they are due to dust trapping at the edges of gas gaps induced by the gravitational torques from embedded planets. If the concentration of solids at the gap edges becomes high enough, it could potentially result in planetesimal formation by the streaming instability. We tested this hypothesis by performing global 1D simulations of dust evolution and planetesimal formation in a protoplanetary disc that is perturbed by multiple planets. We explore different combinations of particle sizes, disc parameters, and planetary masses, and we find that planetesimals form in all of these cases. We also compare the spatial distribution of pebbles from our simulations with protoplanetary disc observations. Planets larger than one pebble isolation mass catch drifting pebbles efficiently at the edge of their gas gaps, and depending on the efficiency of planetesimal formation at the gap edges, the protoplanetary disc transforms within a few 100 000 yr to either a transition disc with a large inner hole devoid of dust or to a disc with narrow bright rings. For simulations with planetary masses lower than the pebble isolation mass, the outcome is a disc with a series of weak ring patterns but there is no strong depletion between the rings. By lowering the pebble size artificially to a 100 micrometer-sized “silt”, we find that regions between planets get depleted of their pebble mass on a longer time-scale of up to 0.5 million years. These simulations also produce fewer planetesimals than in the nominal model with millimeter-sized particles and always have at least two rings of pebbles that are still visible after 1 Myr.

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MRI-active inner regions of protoplanetary discs. I. A detailed model of disc structure

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab920 ◽

2021 ◽

Vol 504 (1) ◽

pp. 280-299

Author(s):

Marija R Jankovic ◽

James E Owen ◽

Subhanjoy Mohanty ◽

Jonathan C Tan

Keyword(s):

Energy Transport ◽

Threshold Temperature ◽

Mass Star ◽

Dust Grains ◽

Solar Mass ◽

Detailed Model ◽

Short Period ◽

Pressure Maximum ◽

Ion Emission ◽

Protoplanetary Discs

ABSTRACT Short-period super-Earth-sized planets are common. Explaining how they form near their present orbits requires understanding the structure of the inner regions of protoplanetary discs. Previous studies have argued that the hot inner protoplanetary disc is unstable to the magnetorotational instability (MRI) due to thermal ionization of potassium, and that a local gas pressure maximum forms at the outer edge of this MRI-active zone. Here we present a steady-state model for inner discs accreting viscously, primarily due to the MRI. The structure and MRI-viscosity of the inner disc are fully coupled in our model; moreover, we account for many processes omitted in previous such models, including disc heating by both accretion and stellar irradiation, vertical energy transport, realistic dust opacities, dust effects on disc ionization, and non-thermal sources of ionization. For a disc around a solar-mass star with a standard gas accretion rate ($\dot{M}\, \sim \, 10^{-8}$ M⊙ yr−1) and small dust grains, we find that the inner disc is optically thick, and the accretion heat is primarily released near the mid-plane. As a result, both the disc mid-plane temperature and the location of the pressure maximum are only marginally affected by stellar irradiation, and the inner disc is also convectively unstable. As previously suggested, the inner disc is primarily ionized through thermionic and potassium ion emission from dust grains, which, at high temperatures, counteract adsorption of free charges on to grains. Our results show that the location of the pressure maximum is determined by the threshold temperature above which thermionic and ion emission become efficient.

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